The invention is directed to a process for the resolution of the racemate S-(carboxymethyl)-(RS)-cysteine, especially for the purpose of recovery of S-(carboxymethyl)-(R)-cysteine. This substance is needed for pharmaceutical purposes and serves for example, as a mucolyticum.
It is known to produce S-(carboxymethyl)-(R)-cysteine by reacting (R)-cysteine--also called L-cysteine--with chloroacetic acid in alkali medium (Armstrong, J. Org. Chem., Vol. 16 (1951), pages 749 to 753).
The (R)-cysteine needed for this purpose as starting material is generally obtained from keratin containing natural materials. For this purpose these are hydrolyzed; the (RR)-cystine set free is separated and reduced to (R)-cysteine (Org. Synth., Vol. 5 (1925), pages 39 to 41); German OS No. 2653332 (and related Scherberich U.S. Pat. No. 4,245,117), Vigneaud, J. Amer. Chem. Soc., Vol. 52 (1930), pages 4500-4504). However, suitable natural materials are only available to a limited extent.
In the synthetic production of cysteine, for example from thiazolines-3 substituted in the 2-position via the corresponding thiazolidin-4-carbonitrile the racemate (RS)-cysteine is formed (German OS No. 2645748). It is known to obtain (R)-cysteine by reacting the (RS)-cysteine with dicyanidiamide to form (RS)-2-guanidine-1,3-thiazolidin-4-carboxylic acid, from this with the help of the copper complex salt of (R)-aspartic acid there is separated the (R)-2-guanidin-1,3-thiazolidin-4-carboxylic acid and subsequently there is split off from this the (R)-cysteine (German AS No. 1795021). This process for the recovery of the (R)-cysteine thus is cumbersome and expensive which is unsuited for use on an industrial scale.